1. Field of the Invention
The present invention relates to a coating for a front face of a printhead in an ink jet printer and, more particularly, to a hydrophobic coating for a front face of a printhead in an ink jet printer, irrespective of whether it is a continuous or a drop-on-demand type, which controls the wetting characteristics of the front face to prevent ink accumulation on the front fact and ensure the directionality of a jet or ink ejected from the various nozzles of the printhead.
2. Description of Related Art
In ink jet printing, a printhead is provided, the printhead having at least one ink-filled channel for communication with an ink supply chamber at one end of the ink-filled channel. An opopsite end of the ink-filled channel has a nozzle opening from which doplets of ink are ejected onto a recording medium. In accordance with the ink droplet ejection, the printhead forms an image on the recording medium.
The ink droplets are formed as ink forms a meniscus at each nozzle opening prior to being ejected from the printhead. After a droplet is ejected, additional ink surges to the nozzle opening to reform the meniscus.
The direction of the ink jet determines the accuracy of placement of the droplet on the receptor medium, which, in turn, determines the quality of printing performed by the printer. Accordingly, precise jet directionality is an important property of a high quality printhead. Precise jet directionality ensures that ink droplets will be placed precisely where desired on the printed document. Poor jet directionality results in the generation of deformed characters and visually objectionable banding in half tone pictorial images. Particularly with the newer generation of thermal ink jet printers having higher resolution enabling printing at at least 300 dots per inch, improved print quality is demanded by customers.
Currently available ink jet printers provide accurate placement of ink droples on a page for only a very limited period of time. The current printers do not maintain high print quality by maintaining the directionality of the ink jet throughout the entire printing lifetime of the printer.
A major source of ink jet misdirection is associated with improper wetting of the front fact of the printhead containing at least one nozzle opening. One factor which adversely affects jet directional accuracy is the interaction of ink previously accumulated on the front face of the printhead with the exiting droplets. This accumulation is a direct consequence of the forces of surface tension, the accumulation becoming progressively severe with aging due to oxidation of the front face of the printhead. Ink may accumulate on the printhead front face due to either overflow during the refill surge of ink or the splatter of small droplets resulting from the process of ejecting droplets from the printhead. When accumulated ink on the front face of the printhead makes contact with ink in the channel (and in particular with the ink meniscus at the nozzle orifice), the meniscus distorts, resulting in an imbalance of forces acting on the ejected droplet. This distortion leads to ink jet misdirection. This wetting phenomenon becomes more troublesome after extensive use of the printhead as the front face either oxidizes or becomes covered with dried ink film. As a result, gradual deterioration of the generated image quality occurs. One way of avoiding these problems is to control the wetting characteristics of the printhead front face so that no accumulation of ink occurs on the front face even after extensive printing. Thus, in order to provide accurate ink jet directionality, wetting of the front face of the printhead is preferably suppressed. This can be achieved by rendering the printhead front fact hydrophobic.
In thermal ink jet printing, a thermal energy generator, usually a solid state resistor, is located in the channels near the nozzle openings at a predetermined distance from the nozzle openings. The resistors are individually addressed with a voltage pulse to momentarily vaporize the ink and form a bubble which expels the ink drople. As the bubble grows, the ink bulges from the nozzle and is contained as the meniscus by the surface tension of the ink. The rapidly expanding vapor bubble pushes the column of ink filling the channel toward the nozzle opening. At the end of the current pulse, the heater rapidly cools, and the vapor bubble begins to collapse. However, because of inertia, most of the column of ink that received an impulse from the exploding bubble continues its forward motion and is ejected from the nozzle opening as an ink droplet. As the bubble begins to collapse, the ink remaining in the channel between the nozzle opening and the bubble starts to move toward the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the necessary momentum and velociy to the droplet in a substantially straight line direction toward the recording medium. However, puddling of ink in contact with the nozzle opening in the front face of the thermal ink jet printhead will cause deflection of the droplet from a sraight line path and, accordingly, misdirection. Therefore, the wetting characteristics of the front fact of the printhead are critical to accurate printing.
All the different types of ink jet printheads include an array of nozzles. Such nozzles of a thermal print head may be formed of silicon wafers using orientation dependent etching (ODE) techniques. The use of silicon wafers is advantageous because ODE techniques can form structures, such as nozzles, on the wafers in a highly precise manner. Moreover, the structures can be fabricated efficiently at low cost. The resulting nozzles are generally triangular in cross-section. Thermal ink jet printheads made by the above-mentioned ODE techniques typically comprise a channel plate which contains a plurality of nozzle-defining channels located in a lower surface thereof bonded to a heater plate having a plurality of resistive heater elements formed on an upper surface thereof, the heater plate being arranged so that a heater element is located in each channel. The upper surface of the heater plate may include an insulative layer which is patterned to form recesses explosing the indiviudual heating elements. The insulative layer is referred to as a "pit layer" and is sandwiched between the channel plate and the heater plate so that the nozzle containing front face may have three layers: 1) the channel plate; 2) the pit layer; and 3) the heater plate.
The heater and channel plates are typically formed from silicon, while the pit layer, sandwiched between the heater and channel plates, is formed from a polymer. Since the front face of the printhead includes these different materials, a coating material, such as a water-repellent material, will not adverse equally well to these different materials resulting in a coating which is not uniformly ink repellent. Thus, it is difficult to provide a surface coating which is uniformly ink repellent over a long period of time for ink jet printheads.
Additionally, the printer is typically used in ink which contains a glycol and water. Glycols and other similar materials are referred to as humectants, which substances promote the retention of moisture. For a coating material to be effective for any length of time, it must both repel and be resistant to glycol-containing inks.
Further, it is difficult to apply a coating to a face of an ink jet nozzle opening. Many materials will not adhere sufficiently to the silicon wafer face. While it is desirable to suppress the wetting property of the nozzle jet surface, it may be undesirable to allow any coating material to enter the channel of the nozzle. If the walls of the channel become coated with ink-repellent material, proper refill of the channel may be inhibited. Refill of each channel depends on surface tension and must be completely in time for the subsequent volley of droplets to be fired. If the refill process is not completely by the time the next droplet is fired, the meniscus may not be flush with the outer edge of the nozzle opening, resulting in misdirection. Further, an incompletely filled channel causes the ink droplet size to vary, which also leads to print quality degradation.
U.S. Pat. No. 4,392,907 to Shirato et al. discloses a method for producing a printhead for ejecting a recording liquid in an action chamber from an orifice in a state of small droplets. The printhead comprises a flat plate provided on a substrate. A protective layer and a filling layer can be provided on the substrate. The protective and filling layers prevent the direct contact of a heating resistor or electrode with the recording liquid or ink, preventing the oxidation of the resistor or electrode and the composition of the ink. The reference provides no disclosure of a coating for the front face of the printhead to ensure the directionality of ink droplets ejected therefrom.
U.S. Pat. Nos. 4,555,062 and 4,583,690 to You disclose ionic surface preparations for nozzles used in spraying ink droplets and ink jet printers. An oppositely charged ionic anti-wetting agent is dissovled in the sprayed fluid to reduce the wetting of the nozzle surfaces. The preparation is not applied to the front face of a printhead to prevent the accumulation of ink thereon.
Thus, the ability to change the wetting charcteristics of the front face of a printhead to simply and effectively ensure directionality of an ink droplet is needed.